1. Field of the Invention
The present invention relates to a photoreactive polymer, and more particularly, to a photoreactive polymer that has an orientation due to photoreaction, and is thermally stable due to a multi-cyclicmulticyclic compound contained therein at as its main chain, and quickly responds to light.
2. Description of the Related Art
Liquid crystal displays (LCDs) are regarded as most competitive displays to be able to for replace replacing cathode-ray tubes since they are lightweight and have a low power consumption power. In particular, thin film transistor-liquid crystal displays (TFT-LCDs) operating by TFTs, which operate using a TFT, exhibit a very quick responding speed because respective pixels operate independently, so that high quality moving images can be realized. Therefore, TFT-LCDs have a wide range of applications, such as notebook computers, wall mount TVs, and the like.
According to a conventional method of manufacturing a color TFT-LCD, a TFT driving device and an ITO transparent electrode are deposited on a glass substrate and then an alignment layer is deposited thereon, thus forming a lower substrate. A spacer formed of sealant is interposed between upper and lower substrates to form a space into which a liquid crystal material is to be injected. A polarized film covers external surface of the glass substrate, and finally, a the liquid crystal material is injected into the space formed between the substrates and hardened to manufacture an LCD cell.
In order to use liquid crystals as an optic switch in a TFT-LCD, liquid crystals needs to be initially oriented in a predetermined direction on a TFT, which is disposed in the most inner portion of a display cell. For this purpose, a liquid crystal alignment film is used.
An alignment layer can be formed by rubbing a polymer resin film, such as a polyimide resin, formed on a substrate using a cloth or the like in a predetermined direction. Alternatively, an alignment layer can be formed by obliquely depositing SiO2. The rubbing treatment is disadvantageous in that impurities may be generated by the contacting, a production yield may decrease due to static electricity, and a contrast may decrease. The obliquely depositing method is not suitable for forming a large display screen and is expensive. Accordingly, the obliquely depositing method is not suitable for a large-size liquid crystal display device.
In order to solve these problems, a non-rubbing process using a photopolymerizable alignment material has been developed. In the non-rubbing process, liquid crystals are aligned by photopolymerisation resulting from optical irradiation. The non-rubbing process is disclosed by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31, 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). Photoalignment refers to a mechanism for aligning of liquid crystals in which a photoreactive group bound to a polymer reacts when exposed to prepolarized ultraviolet rays such that a main chain of the polymer is aligned in a predetermined direction.
A pPhotopolymerizable alignment layer is typically formed of a polycinnamate-based polymer, such as poly(vinyl cinnamate) PVCN and poly(vinyl methoxycinnamate) PVMC. Although the polycinnamate-based polymer has excellent photoalignment characteristics, it is thermally unstable. That is, thermal stability of the alignment layer is dependent on the thermal stability of a polymer used to form the alignment layer and the main chain of a PVCN-based polymer has a glass transition temperature of 100° C. or less. Therefore, the thermal stability of the alignment layer is low.
Meanwhile, Japanese Laid-open Publication No. hei 11˜181127 teaches a method of forming an alignment layer formed of a polymer that containing contains acrylate, metacrylate as a main chain and a photoreactive group, such as a cinnamic acid group, as a side chain and an alignment layer formed using the method. In this case, since a polymer has a low mobility, sufficient alignment characteristics cannot be obtained even when the polymer is exposed to light for a long time. This which is because the photoreactive group contained in the polymer is bound to the main chain of the polymer and cannot react quickly when exposed to a polarized light. Accordingly, a long period of time is required to produce a network polymer and thus process efficiency decreases. In addition, when the alignment treatment is completed for an insufficient time not performed for a sufficient length of time, the produced liquid crystal display device has insufficiently aligned liquid crystals and a low dichroic ratio, and a contrast thereof deteriorates.
Accordingly, there is an increasing need to develop a photoreactive polymer having an excellent thermal stability and an improved photoreaction rate.